Maaser Sheni 4:1-2

If a person transports second tithe produce from a place where it sells at a higher price to a place where it sells at a lower price, or vice versa, he redeems it for the price of where he is. If a person transports grain from the threshing floor to the city (where it sells for more), or wine from the vat to the city (where it sells for more), the extra money he earns is second tithe and the expenses incurred in transporting his wares comes out of his pocket.

Ma’aser Sheini 4:2

Second tithe may be redeemed at wholesale prices: what a shopkeeper pays for his goods, not what he charges for them; what a money-changer pays, not what he charges. Second tithe may not be redeemed in a pile, estimating its value. If the price is known, it may be redeemed on the word of one appraiser. If the price is not known, it must be appraised by three. Examples of things that do not have set prices are wine that is turning, produce that has started to go bad, and coins that have rusted.